When stainless steel is welded, the resulting join and its surrounding area are often discoloured and this area (if untreated) then becomes susceptible to corrosion. This weld burn that appears, after welding with a TIG (ie, “Tungsten Inert Gas” arc welding) welder for instance, therefore normally has to be cleaned, and this is often carried out using an electro-chemical cleaning system in recent times. That cleaning system involves using an electro-cleaning unit that applies a low voltage, high current, flow in a circuit with the metal connected as the anode. A tool, providing the cathode, is rubbed over the heat tinted sections of the weld and its surrounding metal, with the aid of an electrolytic weld cleaning fluid, to complete the circuit. The tool heats and passivates the metal surface, and removes the unwanted discolouration. The passivation treatment restores its natural protection against corrosion to the stainless steel.
In addition, when there is a need to provide a bright mirror finish to the stainless steel, which is particularly important when the steel article is used in the food industry, or when a decorative appearance is needed, for example, then a similar system for electro-polishing is also utilised using the same type of electrolytic polishing fluid.
Traditionally in the past, weld discolouration was removed using a pickling paste that usually consisted of a composition of nitric acid and hydrofluoric acid. This paste was manually wiped on to the stains on the welds and the surrounding area, left in place until the stain had been removed, and then washed off. These acids are dangerous and difficult to apply and remove. The waste and fumes that result have to be handled carefully, being dangerous to breathe in or have skin contact with, and the waste liquid generated is environmentally hazardous to dispose of.
More recently, electro-chemical cleaning or polishing has been utilised to achieve this same purpose, and this has been found to often give a better result. Electro-chemical cleaning (or “electro-cleaning”) also produces a better finish to the metal, since pickling will roughen the surface while electro-cleaning smoothes it.
Electro-cleaning (or polishing) basically involves using an electrical power source that is connected in a circuit to the steel surface which becomes the anode, and with an electrically conductive “wand” tool comprising the cathode. The wand can be in the form of a copper electrode, surrounded by a nylon sponge or felt pad which is soaked with an electrolyte solution. Until the current invention, this electrolyte solution was normally an aqueous solution of phosphoric acid (ie, orthophosphoric acid H3PO4). This wand is used to rub and corrode away the top surface layers removing the discolouration and iron oxide surface imperfections left from the welding. A worker holds the wand by an insulated handle and presses it over the tinted area until the normal stainless steel sheen is returned and the discolouration removed.
More recently, the applicants for the present invention (Ensitech), have developed an improved electro-cleaning system that utilises a conductive carbon bristled brush (the “TIG Brush™”) that is used in place of the felt or nylon pad type of wand. There is a power source that produces a low voltage, high current, output that is used to remove the discolouration from stainless steel TIG welds when the electrical current is flowing in the circuit. The brush is dipped in the electrolytic cleaning solution and then the tips of the conductive bristles are dragged across the tinted surface area. The bristles allow the brush to penetrate into constricted areas, and any crevices or pits in the metal, giving an excellent result. The electrical current passing via the brush is thought to create a high temperature area on the surface of the steel that hastens the deoxidising action of the cleaning fluid. The iron oxides on the surface are dissolved to release free ferric and ferrous ions and oxygen gas.
In addition, the welds in the stainless steel and the surrounding area then require passivation to restore its resistance to corrosion. Stainless steel is an alloy of iron that contains a significant amount (more than 10% and commonly 13% to 27%) of chromium. The “stainless” quality of this steel is caused by a thin durable film of chromium oxide that forms on its surface, which will be naturally formed by the oxidation of the stainless steel alloy with air or water. Welding disrupts this protective coating, and the iron oxides that instead form on the surface cause the unwanted heat tint staining, as well as allowing corrosion and pitting to occur subsequently. Passivation restores the protective chromium oxide surface layer by stripping away any iron oxides on the surface and simultaneously oxidising the chromium atoms present to form the new surface layer.
Passivation generally is the process of removing the unwanted iron particles on the surface and oxidising the chromium atoms that replace them, using an oxidising agent like an acid, or by contact with air or water. The Standard “ASTM A380”, defines passivation to be “the removal of exogenous iron or iron compounds from the surface of stainless steel by means of a chemical dissolution, most typically by a treatment with an acid solution that will remove the surface contamination, but will not significantly affect the stainless steel itself.” It also describes passivation as “the chemical treatment of stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of enhancing the spontaneous formation of the protective passive film.”
For a long time, pickling and passivation of stainless steel involved using acids, especially nitric acid, or a mixture or nitric and hydrofluoric acids, or citric acid for stainless steel to be used in the food industry. More recently with both types of the electro-cleaning units mentioned above, an electrolytic and oxidising cleaning fluid has been used that consists of a solution of phosphoric acid in water, together with a small quantity of fragrance and colouring. This solution is acidic, with a low pH of approximately pH 1 to 3. Probably because of the historic use of acids, and their effectiveness with pickling and passivation, welding cleaning solutions have all been acidic in nature.
Some passivation and cleaning procedures used in the past have also used strongly basic or alkaline solutions with a high pH, for example using aqueous solutions of sodium hydroxide, often in sequence with the acid treatments.
These acidic (or basic) welding fluids create environmental problems with their disposal after use, and have to be used with great care by the workers carrying out the cleaning process to avoid splashing the fluid on to skin or into eyes.
It has now been found that using a weld cleaning fluid that has a close to neutral pH of around 7, can avoid at least some of the problems arising from the acidic or alkaline fluids used in the past. Surprisingly, it has been found that neutral pH weld cleaning and passivation fluids can give superior results over the low (or high) pH fluids. Additionally the cleaning and passivating treatment can proceed more quickly than with the prior known fluids, giving an economic benefit as well as exposing workers to less of the fumes that are given off during the cleaning process.